Most beginners don't aware that without using protection, their app would easily manipulate and reversed. That is very dangerous for new developers. This issue can threatened new developers for security about their app. For solving this issues we can use optimizers. So we can start from basic and free optimizer ProGaurd.
ProGaurd obfuscating the name of classes, fields and methods that provides minimal protection against reverse engineering. It makes hard for reverse-engineers to undestand, copy or manipulate the code.
ProGuard also minifies project code. It makes java and Android applications smaller and also increase the speed fo app.
It's absolutely free and comes bundle with Android SDK.